Travelling wave oscillator



March 17, 1959 P. M. LALLY 2,878,412

TRAVELLING WAVE OSCILLATOR V Filed Sept. 4, 1953 2 Sheets-Sheet 1 INVENTOR Pl/lL/P M. L/JLLY ATTORNEY March 17, 1959 P. M. LALLY TRAVELLING WAVE OSCILLATOR Filed Sept. 4, 1953 Fig. 3;

2 Sheets-Sheet 2 INVENTOR PH/L M. L Y BY 102 United States Patent TRAVELLING WAVE OSCILLATOR Philip M. Lally, Levittown, N. Y., assignor to Sperry Rand Corporation, a corporation of Delaware Application September 4, 1953, Serial No. 378,509

11 Claims. (Cl. 315-35) This invention relates to a microwave oscillator, and more particularly, is concerned with a tunable travelling wave tube type oscillator.

It is the general object of this invention to provide an improved oscillator for operation in the microwave frequency region, the oscillator being characterized by its broad tuning range, frequency stability, and simple output coupling arrangement.

Another object of this invention is the provision of an oscillator utilizing interaction between an electron beam and an electromagnetic field in which the interaction region is positioned entirely within a hollow wave guide transmission line.

Another object of this invention is to provide a microwave oscillator in which the frequency of oscillation is substantially independent of variation in load impedance.

Another object of this invention is the provision of a microwave oscillator which may be either electrically or mechanically tunable, or both, depending on its mode of operation.

These and other objects of the invention which will become apparent as the description proceeds are achieved by the provision of an electron discharge apparatus including an electron gun for producing a stream of electrons, and a collector. A hollow rectangular wave guide section extends between the gun and the collector, the longitudinal axis of the wave guide being perpendicular to the'electron stream with the narrow walls of the wave guide having apertures therein to permit passage of the electron stream across the interior of the wave guide section. Sealed windows are provided on either side of the electron stream region to permit evacuation in this region. An interaction structure is positioned within the wave guide along the path of the electron stream, the interaction structure including a pair of conductive members secured to the inner surfaces of the broad walls of the wave guide, the members having opposed parallel surfaces, with a plurality of conductive fins extending from these surfaces, the fins projecting towards the opposite member. The fins projecting from each member are spaced between a pair of fins projecting from the opposite member so that an interleaved structure is provided. Each of the fins has an opening therein positioned along the axis of the stream for permitting passage of the electron stream through the interaction structure. Mechanical tuning is achievedby providing mechanical adjustment of the spacing between the two parallel surfaces of the conductive members.

For a better understanding of the invention, reference should be had to the accompanying drawings, wherein:

Fig; 1 is a plan view, partially in section, of the oscillator tube;

Fig. 2 is a sectional view taken substantially on the line II--II of Fig. 1;

' Fig. 3 is a perspective view of the tube of Fig. 1; and

Fig.-4 is a fragmentary cross-sectional view taken sub stantially on the line IVIV of Fig. 2.

Referring to the drawings, the numeral indicates ice generally an electron gun of conventional design including a cathode 12, a heater 14, and accelerating anode 16. These electrodes are supported from a base 18 which is provided with pins 20 and suitable internal connections (not shown) by means of which heater current may be supplied to the heater 14 and a negative cathode potential may be applied between the cathode 12 and anode 16, the anode being normally at the ground potential of the envelope of the tube.

The electron gun 10 is secured in sealed arrangement to the narrow wall 22 of a rectangular wave guide section, indicated generally at 24, by means of an insulating glass seal 26, mounting ring 28, and mounting flange 29. An aperture 30 through the narrow wall 22 permits, passage of the electron stream from the cathode 12 transversely across the center of the wave guide section :24. An aperture 31 in the opposite narrow wall 32 of the wave guide section 24 permits the electron stream from the cathode 12 to impinge on a collector, indicated generally at 33, including a collector electrode 44. The collector 33 is supported in hermetically sealed relation to the wave guide section 24 by means of a glass seal 34 and associated mounting rings 36and 38, the ring 36 being supported by an annular mounting flange 40 secured to the narrow wall 32 of the wave guide section 24. The collector 33 is normally maintained at the ground potential of the wave guide section 24. Cooling discs 42 maybe provided surrounding the collector electrode 44. Magnetic focussing of the electron stream to maintain. a narrow beam may be provided by an external magnet (not shown) in conventional manner, the poles, of the magnet being in the form of yokes which may be clamped around the mounting rings 28 and 40. For an example of magnetic focussing of an electron beam in similar types of high frequency tubes see pending application Serial No. 117,187 filed in the names of Rich and Wang, now U. S. Patent 2,687,490, issued August 24, 1954.

To provide an evacuated envelope for the electron stream, mica windows 46 and 47 are positioned across the ends of the wave guide section 24. The mice. windows are clamped in position by adjacent wave guide sections 48 and 50 respectively, each of the wave guide sections 48, 24, and 50 being provided with flanges 52, 54, 56, 58, 60 and 62 at the respective ends thereof. Flanges 54 and 56 and flanges 58 and 60 are bolted together as by bolts 64 to secure the wave guide sections together. Suitable Wax is preferably applied between the surfaces of each mica window and adjacent surfaces of the flanges to provide an airtight seal at the ends of the wave guide section 24.

Situated within the wave guide section 24 along the axis of the electron beam are a pair of conductive blocks 66 and 68 having opposed flat parallel surfaces 70 and 72 respectively. The block 66 is secured to the upper broad wall 74 of the wave guide section 24 and is provided with a plurality of conductive fins 76 preferably integral with the block 66, projecting from the surface 70 as to extend across the axis of the electron stream. Each of the projecting fins 76 is provided with, a slotted opening 78, the slots lying along the axis of the electron stream to permit the passage through the fins 76 of the electrons as they pass from the electron gun 10 to the collector 32.

The opposing block 68 is similarly provided with a plurality of projecting fins 80 which extend between the fins 76 so as to form an interleaved or inter-digital structure. The fins 80 are similarly provided with slots 82 to permit passage of the electron stream through the interleaved fins. The width of the fins 76 and 80, as measured in a direction parallel to the longitudinal axis of the wave guide, is not critical but should be theminimum required to. achieve a wellgdefined field in the regions between the fins. The number of fins depends on the desired frequency of operation and beam voltage, as will hereinafter become apparent.

To,:provide mechanical.tuning,,.the ,block 68 is made .adjustablerelative to..the block .66. This adjustment is provided bymeans of a rectangular opening83 in the broad wall 84 of the rectangular wave guide section 24 through which the lowerend of the block 68 extends. A choke member 86 is provided on the outside of the rectangular wave guide section 24, the choke member 86 having an opening 87 registering with the opening 82 in the rectangular wave guide and being provided with annularchoke slots 88 andl90.

The block 68 is adjustively supported by means of a rod" 92 having a threaded member 94 secured to the end thereof opposite tov the end of block 63. The member 94 includesaiflange portion 96, abellowsi98 extending between the, chokemember 86 and the, flangefiti toprovide a vacuum seal. The threaded member 94 isclarnped by means of. locking nuts 100 and,102 to a bracketlttd rigid- ,ly supported asby being brazedor otherwise secured to the choke member 86. .By loosening and-adjusting the position of the locking nuts1100 and 102, the distance between the surfaces 70 and .72 of the blocks 66 and 625 can be readily adjusted.

As is hereinafter pointed out in the. discussion of the operation of the tube, it is desirable that one end :of the wave guide transmission line be terminated inan adjustable short circuit. This is provided by a short circuiting plunger106. Adjustment of the plunger 106 within the wave guide 48is provided by means of a micrometer adjustment, indicatedgenerally at 108,.having a calibrated adjusting .knob 110. Akey 112 engages a groove inthe micrometer shaft 114. to prevent rotational movement of the plunger 106. The entire adjustable short circuit assembly is supported in position by a flange 116 bolted as by means. of bolts 118 to the flange 52 on the wave guide section 48.

According to the preferred mode of operationofthe oscillator as abovedescribed, the oscillator functions substantially as abackward-Wave travelling wive oscillator. As in the conventional backward wave oscillator, the electron beam in passing. through the interaction structure excites an electromagnetic wave which is propagated back toward the source. The interaction structure therefore propagates electromagnetcenergy, and acts as a conventionalfslow wave propagating structure. The frequency offlthiswaveis a. function of of the electron velocity and therefore a function of the beam voltage on the anode. To get energy to transfer from the electron stream to the field, the electron velocity must be such that it moves from one fin to the next in a period of time slightly less thanthe period of one half cycle of the electromagnetic wave less the time it takes the wave to .travel that distance. The frequency ofthe field will adjust itself according to the electron velocity to maintain this condition for oscillation.

' In the conventional backward-wave oscillator, the interaction or slow Wave propagating structure must be made quite long so that there is sufficient distributed feed back to. startoscillation. In the present oscillator, the slow Wave propagating structure is relatively short. However, it is terminated at the ends thereof in an open circuit which. acts as a reflective termination. If thelength of the slow wave propagating structure is an integral num ber of half wavelengths long, waves traveling in the same direction reinforce each other and are continuously acted on by the electron beam. Oscillation results with much shorter length of interaction structure because of the additional feedback introduced .by the reflective terminations.

Thus oscillation is sustained where the interaction structure is resonant, namely, where thelength is an integral number of half wavelengths longs, 'which-condition may be expressed mathematically as L= /2.n (1) The wave length N; in turn may be expressed in terms of the phase velocity w of the fundamental and the frequency as so that the frequency of oscillation, by equating (l) and is This equation may be used to calculate the frequencies at which the tube will. oscillate for a .given phase velocity and axial length of the interaction or slow wave propagating structure. The phase velocity is substantially independent of frequency and may be determined rather closely in terms ,of a wave travellingat the, speed oflight along the serpentine path defined by. the interleaved fins.

Thus the phase velocity is substantially equal to the velocity of light multiplied by the ratio of the serpentine path length to the axial path length, or

v..=.c o)

where S is the length of the serpentine path and cis the velocity of light.

'Thebeam voltage, i. e, the applied potential between the cathode'12 and the anode 16, depends on the desired frequency and the spacing between the fins. The voltage to give oscillation at a given .design fr quency can be determined in the followingmanner: .As pointed out above, to get oscillation, the electron should traverse the distance from one fin tothe nextv in a timeslightly less than ,the period of one. half cycle of the. electromagnetic Wave less the time it takes the wave to travel that distance, or expressed mathematically d l d Since the electronvelocity is related to the beam voltage 1' by/the equation where eis the charge and m is the mass of an electron, the .value of V for a given frequency f and fin spacing dcan be readily calculated from Equations (6),.and (7). Actually the voltageshould beslightly greater than this calculated figure, but. oscillation has been found to take place over a 20%. voltage range with the value calculated by the above equations-representing the lowerlimit.

With the interaction structure functioning as a resonant device, the beam voltage has negligible effect on. the frequency asvlong as the electron velocity is held-within the 20% voltage spread in which interaction takes-place between the electron beam and the electromagneticqfield for a particular mode of oscillation. Change in-voltage does affect the power output.

Themodenof oscillation as represented by the-integral number n inthe above equations canbe changedby shifting the beam voltage to a value where interaction takes place for a different wavelength at which the slow. wave propagating structure isresonant- Thus, by changing the beam voltage in predetermined steps, the mode-of oscillation can be changed with a corresponding jump in the frequency. It should :bemoted thatthe lowestmode of oscillation may be-belowxthecutoii frequency vofithe smears a change in the velocity of the fundamental with a resultant change in frequency. If the phase velocity is changed very much by the mechanical tuning, there must be an accompanying change in the beam voltage, however, for interaction between the beam and the field to sustain oscillation. Continuous tuning is thus possible over a reasonably wide frequency range by tuning modes mechanically and shifting modes electronically. Because the frequency is substantially independent of variations in beam voltage in a given mode of oscillation, the frequency of the output is quite stable.

Coupling of energy from the transverse propagating structure to the wave guide is provided by fringing field effects existing along the sides of the wave propagating structure. For this reason, coupling to the wave guide islight, and loading has negligible effect on the frequency of oscillation.

There is a second mode of operation by which the tube structure above described can be made to oscillate. Assume for the moment that the wave guide section is excited by a microwave signal of a certain frequency. At a given instant, then all the fins projecting from one side of the guide in the slow wave propagating structure will be at substantially the same potential with respect to the fins projecting from the opposite side of the guide. If an electron travels from fin to fin in one half cycle of the energy propagated along the wave guide, it experiences a net electric field in one direction, since the effect of the alternating potential at the fins exactly cancels the alterations in the field. Thus the electron travels at the same speed as an apparent travelling wave. This apparent wave can be thought of as a spatial harmonic of a wave travelling with infinite phase velocity.

If a beam of electrons is sent through the interaction structure at a speed corresponding to exact synchronism with the velocity of this apparent spatial harmonic, just as many electrons are accelerated as are decelerated so that there is no net transfer of energy between the electron stream and the electromagnetic field. However, if the beam is travelling somewhat slower than the spatial harmonic wave, the beam will gain energy from the field, whereas if the beam is travelling somewhat faster than the spatial harmonic wave, the electronic stream will give up energy to the electromagnetic field. In the latter case, if the energy gained is sufiicient to supply the loss in the tube and in any loads on either end of the output wave guide transmission line, the system is self-oscillatory. The frequency then is determined by the beam voltage. As the voltage is changed, the oscillation frequency changes so as to maintain the phase velocity of the apparent spatial harmonic travelling wave slightly greater than the electron velocity.

Normally the oscillator is tuned to oscillate at a frequency where the interaction structure is not resonant, since at such frequencies where the interaction structure is resonant, electronic tuning becomes ineffective and the oscillator goes into resonant oscillation. Resonant oscillation can be reduced or eliminated by providing energy absorbing non-reflective terminations at either end of the interaction structure, as by inserting attenuating mate rial between the fins. This prevents reflection of electromagnetic waves along the interaction structure parallel to the electron stream axis, so that backward-wave oscillation can not take place.

The short circuit provided by the tuning plunger is ad justed to balance out the shunt susceptance introduced by the interaction structure in the wave guidetransmission line, so that the oscillator can be matched to the transmission line. By changing the position of the short at the same time the beam voltage is varied, a wide range of frequency may be achieved over which the oscillator can be tuned.

From the above description it will be seen that the various objects of the invention have been. achieved by the provision of a unique tube structure which functions as a microwave oscillator. The interaction structure along the beam is extremely short compared to that in known travelling wave tube type oscillators, which permitsmounting within a transverse wave guide transmission line. The result is a relatively compact structure having a convenient output coupling means in the transverse wave guide. Electronic or mechanical tuning is available depending on the mode of operation.

In the preferred mode of operation, namely, where the interaction structure acts as a resonant circuit, electromagnetic energy is actually propagated along the structure parallel to the electron stream and transversely of the wave guide. Thus the interaction structure operates as a conventional slow wave propagating wave guide, that is, there is a phase shift along it. However, in the alternative mode of operation there is no phase shift along the interaction structure, all the fins projecting from one side being in the same phase as determined by the fundamental wave propagated by the wave guide transmission line 24.

While tuning out the shunt susceptance of the interaction structure in the wave guide transmission line is essential in the non-resonant mode of oscillation, it is not essential that the transmission line be terminated in a short circuit at one end in the resonant mode of operation. For this reason the short circuit is shown mounted externally of the vacuum envelope of the tube, so that it may readily be detached if desired. However, it may as well be positioned within the vacuum envelope where the tube is to operate primarily in the non-resonant mode.

The mechanical tuning is of no significance in the nonresonant mode of oscillation. Adjustment of the spacing between the two blocks of the interaction structure may be eliminated therefore if desired, where operation is nonresonant or at a non-tunable resonant frequency.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Electron discharge apparatus comprising an electron gun and collector, the electron gun producing a stream of electrons directed along an axis, a hollow rectangular wave guide section having a pair of narrow walls and a pair of broad walls, the wave guide section extending between the gun and collector the axis of the electron stream being perpendicular to the longitudinal axis of the wave guide section, each of the narrow walls of the wave guide section having an aperture therein to permit passage of the electron stream across the interior of wave guide section, and means positioned. in the wave guide section transverse said longitudinal axis and along the axis of the electron stream including a pair of conductive blocks having spaced parallel surfaces and a plurality of conductive fins projecting from said surfaces, the

fins projecting from one block extending between the fins as'rsnia e of the blocks to vary the spacing between said parallel surfaces.

v2. Electron discharge apparatus comprising, an electrongun and ,collector, the electron gun producing a streamof electrons directedlalong an axis, a hollow rectangular wave guidessection having a .pairof narrow-walls and a pair of broad walls, the wave guide section extendingbetween. thegun ,and collector, the ,axisofthe electron stream being perpendicular ,to the longitudinal axis of thetwave guide section, each. of the narrowwalls of the wave. guide section having,an.;aperture,therein to permit passagetof the electron stream across the interior of wave guide section, and means positioned in the wave guide. section-,transverse'said longitudinal axis and along the, axis ofthe electron streamincludinga pair of conductiveblockshaving:spaced parallel surfaces and a pluralityofi conductive fins projecting from said surfaces, the fins projectingfrom one block extending between the fins projecting from the;,other bloek,; whereby an interleaved structure is providedacross the interior. of said wave guide section, eachof the fins -having an opening therein, the openings being positioned along the axis of the electron stream to,permitpassagethereof through said means.

3. Electron discharge apparatus comprising an electron gun and collector, the electron gun producing a stream of electrons directed along an axis, a hollow rectangular waveguide section having a pair ofnarrow walls and, a pair of broadwalls the wave guide section extending between the'gun'and collector, the axis of the electron stream being perpendicular to the longitudinal axis of the waveguide section, each of the narrow walls of the waveguide section having an aperture therein to permit passage of the-electron stream across the interior of wave guide section, .and means positioned in the-waveguide section transverse said longitudinal axis and along the axis of the electron stream including a pair of conductive blocks having spaced parallel surfacesand a plurality of conductive fins projectingfrom said surfaces, each of the fins having an opening therein, the openingsbeing positioned along the axis of the electron stream-topermit passage thereof through said means.

4. Electron discharge apparatus comprising an electron gun and collector, the electron; gun producing a,.stream of electrons directed along an axis, a hollow rectangular Wave guide section extending-between thegun and collector, the axis of the electron stream being per-pendicular to the longitudinal axis of the waveguide-section, the narrowside walls of the wave guidesection-having apertures therein'to permit passage of the electron stream-acrossthe interior of said wave guide section, and

slow wave propagatingmeans-positioned in-the wave,

guide section along the axis of the electron stream, and transverse the longitudinal axis of said waveguide section.

5. A microwaveoscillator comprising a hollowwavc guide transmission line having a longitudinal axis of propagation, an adjustable short circuit tet mination at one end of the transmission line, means;for directingan electron stream transversely across the interior of-the hollow wave guide transmissionline along an axis perpendicular to said axis of, propagation, nieanstransparent to microwaves in the transmission line on either side of the region of the electron stream for providing an airtight seal whereby the region ofthe electron streanrmay be evacuated, and slow wave propagating means positioned within the transmission line along the axisof the electron stream transverse the longitudinal axis of propagation of said wave guide transmission line, said means including a pair of spaced conductive members on either side of the electron stream and inter-digital conductive fins projecting from said members and extending across the electron stream within the interior of said hollow wave guide transmission line, the fins having openings therein aligned along the axis of the electronstream for passage of the stream through the inter-digitalfins.

6. A microwave oscillator comprising a hollow wave guide transmission line having: a longitudinal axis of propagation,. an 5 adjustable. short circuit termination at one: end of the transmission. line, means for directing .an electron stream transversely across the ,interior of the hollow waveguide transmission: line along an axis .perpendiculantosaid. axis of propagation, means transparent toimicrowaves; in the transmission line on either side of they region pofthe electron stream for providing an airtightseah whereby the region of the electron stream may be .evacuated,,.and means positioned within the transmission line along the axis of the electron streamtransverse the longitudinal axis of propagation of said wave guide transmissionline, said means including a pair of spaced conductive members, on eitherside of the electrontstream and inter-.cligital conductive fins projecting from ,said members and extending across the electron stream within the interior of saidhollow wave guide transmission line, the fiushavingopenings therein aligned along theaxis of the electron stream for passage ofthe streamthrough the interrdigital fins.

7., A microwave oscillator comprising ahollowwvave guide transmission line having a longitudinal axis of prepagatiomimeans 'fordirecting an electron stream transversely across the interior of the hollow wave guide transmission line along an axis perpendicular to said 'axisof propagation means in the transmission line on either side of the region of the electron stream for providingan airtight seal whereby the region of the electronst-ream may beevacuated, and means positioned within the transmission line. alongthe axis of the electron stream transverse the'longitu'dinal axis of propagation of saidwave guide transmission line, said means including a pair of spacedconductive members on either side ofthe electron stream and inter-digital conductive fins projectingfrom said members and extendingacross the -,clectron stream withinjthe interior of said'hollowwave guide transmission line, the fins having openings therein aligned along the axis of the electron stream for, passage of the stream through the inter-digital fins.

8. A microwave oscillator comprising a hollow wave guide transmission line having a longitudinal axisof propagation,=means ,for directing an electron stream transversely across the interior of the hollow wave guide transmission line along an axis perpendicular to said axis of propagation, and means positioned within the transmis sion-lineialong the axis of the electron stream transverse said longitudinal axis of propagation of said wave guide transmission line, said means including a'pair ofspaced conductive-members on either .side of the electron stream, and conductive fins projecting from said members'and extending,across the electron streamwithin the interior of said hollow wave guide. transmission line, the fins having openings therein aligned along the axis of the electron stream for passage .of the stream through the fins.

9. Ainicrowave oscillator comprisingva hollow wav guide transmission line having a longitudinal axis of propagation, said hollow wave guide transmission line, having wide andnarrow cross sectional dimensions, means fondirecting an electron stream, transversely across the interior of the hollow wave guide transmission line along an axis perpendicular to. said axisof propagation, first and second apertures located in opposite wall portions of said hollow :wave guide transmission line along a line parallel with the wide cross sectional dimension thereof forpassageof said electron beam through said apertures, and slow wave propagating means positionedwithin the transmission line and extending transversely of the longitudinalaxis of propagation of said transmission line and along the axis of the electron stream.

10. A microwave oscillator, comprising meansfor directing an electron stream along an axis, slow Wave propagating means having a longitudinal axis positioned along the axis of the electron stream'for passage of said stream through said propagating means,the slow wave propagating means being substantially an integral number of half wavelengths long at the frequency of operation and having open circuited mismatched reflective terminations of substantially unity reflection coeflicient at both ends whereby the slow wave propagating means is resonant at the operating frequency, and means along substantially the entire length of said slow wave propagating means for coupling of microwave energy therefrom for propagation in a direction perpendicular to the axis of the electron stream, said last-named means comprising a rectangular wave guide having its wide cross sectional dimension parallel with the axis of said electron stream.

11. A microwave oscillator comprising means for directing an electron stream along an axis, slow wave propagating means positioned along the axis of the electron stream, said last-named means including a pair of conductive members having spaced parallel surfaces on op- 10 posite sides of said axis and a plurality of inter-digital conductive fins projecting from said surfaces, the fins having openings therein aligned along the axis of the electron stream for passage of the stream. through the interdigital fins, and means for adjustable positioning at least one of the members in a direction transverse said axis to vary the spacing between said parallel surfaces.

References Cited in the file of this patent UNITED STATES PATENTS 2,505,529 Crawford et al Apr. 25, 1950 2,552,334 Linder May 8, 1951 2,651,738 Ebers Sept. 8, 1953 2,653,270 Kompfner Sept. 22, 1953 2,654,047 Clavier Sept. 29, 1953 2,745,984 Hagelbarger et a1. May 15, 1956 

